We propose to continue our studies of the interactions of artificially generated lipid vesicles with mammalian cells. Asymmetric vesicles will be prepared using appropriate donor lipids and phospholipid exchange proteins. Such vesicles will permit a testing of various models for membrane fusion and lipid exchange processes. Asymmetric vesicles will also be used to perturb the natural membrane lipid asymmetry, and the persistence of this asymmetry with time will be investigated. We plan to identify and characterize the cell surface protein in Chinese hamster fibroblasts and mouse thymocytes responsible for binding of lipid vesicles to the cell membrane. We propose to study natural membrane asymmetry by following the action of phospholipid exchange proteins on inside-out and right-side-out plasma membrane fragments isolated from cells grown on radiolabeled precursors for membrane lipids. BIBLIOGRAPHIC REFERENCES: Pagano, R.E. and L. Huang. 1975. Interaction of Phospholipid Vesicles with Cultured Mammalian Cells. II. Studies of Mechanism. J. Cell Biol. 67: 49-60. Huang, L., K. Ozato and R.E. Pagano. 1976. Interaction of phospholipid vesicles with mouse thymocytes in vitro. Fed. Proc. 35: 1703.